Recently, much attention has been focused on GaN-based compound semiconductors (e.g., InxAlyGa1xe2x88x92xxe2x88x92yN, wherein x+yxe2x89xa61,0xe2x89xa6xxe2x89xa61, and 0xe2x89xa6yxe2x89xa61) for blue, green, and ultraviolet light emitting diode (LED) applications. One important reason is that GaN-based LEDs have been found to exhibit efficient light emission at room temperature.
In general, GaN-based LEDs comprise a multilayer structure in which n-type and p-type GaN-based semiconductor layers are stacked on a substrate (most commonly on a sapphire substrate with the n-type GaN-based semiconductor layer in contact with the substrate), and InxGa1xe2x88x92xN/GaN multiple quantum well layers are sandwiched between the p-type and n-type GaN layers. A number of methods for growing the multilayer structure are known in the art, including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
In general, p-type GaN-based semiconductor layers formed by growth methods, such as MOCVD, behave like a semi-insulating or high-resistive material. This is thought to result from hydrogen passivation caused by hydrogen that is present in the reaction chamber complexing with the p-type dopant and thus preventing the dopant from behaving as an active carrier. Typically, p-type GaN-based semiconductor materials are thermally annealed to activate the p-type carriers. However, even after thermal annealing, the resistivity of p-type GaN-based semiconductor materials remains relatively high making it difficult to form a satisfactory ohmic contact with the material. In addition, there are few metals with a high work function comparable to the band gap and electron affinity of gallium nitride and that will form a low resistance interface with gallium nitride. A good ohmic contact to gallium nitride is desirable because the performance of gallium nitride-based devices, such as the operating voltage is strongly influenced by the contact resistance.
Sapphire is generally used as the substrate for GaN-based LEDs because it is inexpensive and GaN-based semiconductor layers grown on a sapphire substrate are reasonably free of defects. However, sapphire is electrically insulative. Thus, electrodes cannot be mounted on the sapphire substrate, but must be formed directly on the n-type and p-type GaN-based semiconductor layers. In addition, since p-type GaN-based semiconductor layers have only moderate conductivity, a p-electrode typically is formed to cover substantially the entire surface of the p-type GaN-based semiconductor layer in a GaN-based LED in order to ensure uniform application of current to the entire layer and obtaining uniform light emission from the LED. However, this geometry requires that the light emitted by the LED be observed through the sapphire substrate or through a transparent p-electrode. Typically, light-transmitting electrodes transmit only 20 to 40% of the light emitted from the LED. Although sapphire has a high transmission coefficient, observation of the light emitted from the LED through the sapphire substrate requires a complicated packaging step. Thus, in order to decrease the cost of manufacture and increase the efficiency of GaN-based LEDs, it is desirable to develop p-electrodes that have improved light transmission.
The invention is an improved electrode for a p-type gallium nitride based semiconductor material that includes a layer of an oxidized metal and a first and a second layer of a metallic material. The first metallic layer has a first surface in contact with a p-type gallium nitride based semiconductor material and a second surface in contact with a first surface of the oxidized metal layer. The oxidized metal layer has a second surface in contact with a surface of the second metallic layer. Preferably, the electrode is light transmissive. In one embodiment, the oxidized metal layer is nickel oxide and the first and second metallic layers are gold.
The electrode of the invention can be used to form a semiconductor device, such as a light-emitting diode (LED) or a laser diode (LD). The semiconductor device includes a substrate having a first major surface. Over the first major surface of the substrate is a semiconductor device structure that includes an n-type gallium nitride-based semiconductor layer, and a p-type gallium nitride-based semiconductor layer over the n-type semiconductor layer. A first electrode is in electrical contact with the n-type semiconductor layer, and a second electrode in contact with the p-type semiconductor layer. The second electrode includes a layer of an oxidized metal and a first and a second layer of a metallic material. The first metallic layer has a first surface in contact with a p-type gallium nitride based semiconductor material and a second surface in contact with a first surface of the oxidized metal layer. The oxidized metal layer has a second surface in contact with a surface of the second metallic layer. Preferably, the second electrode is light transmissive and forms an ohmic contact with the p-type semiconductor layer. The oxidized metal layer includes metal oxides such as nickel oxide and zinc oxide. The oxidized metal layer is preferably nickel oxide. Typically, the layer of metallic material includes metals such as gold, nickel, palladium, platinum, silver, and combinations thereof. In one embodiment, the first and second metallic layers are substantially the same. Preferably, the first and second metallic layers are gold.
To prepare semiconductor devices utilizing the electrode of the invention, a substrate having a first major surface is provided, and a semiconductor device structure is provided over the first major surface of the substrate. The semiconductor device structure includes an n-type gallium nitride-based semiconductor layer and a p-type gallium nitride-based semiconductor layer over the n-type semiconductor layer. An electrode is formed that is in electrical contact with the n-type gallium nitride-based semiconductor layer. Three or more metallic layers are formed over the p-type semiconductor layer such that at least one metallic layer is in contact with the p-type semiconductor layer. At least two of the metallic layers are then subjected to an annealing treatment in the presence of oxygen to form an electrode in contact with the p-type gallium nitride-based semiconductor layer. Generally, the annealing treatment is conducted at a temperature that is about 400xc2x0 C. or more but below the decomposition temperature of the GaN-based semiconductor layers. Preferably the annealing temperature is in the range of between about 400xc2x0 C. and about 550xc2x0 C. In one embodiment, the annealing treatment is preformed in an environment that includes oxygen and nitrogen. After the annealing treatment, the electrode formed is typically light transmissive. In one embodiment, prior to the annealing treatment, the electrode has a first metallic layer having a first surface is formed in contact with the p-type semiconductor layer, and a second metallic layer having a first surface is formed in contact with a second surface of the first metallic layer. After annealing the second metallic layer is substantially oxidized to a metal oxide. A third metallic layer is then formed in contact with a second surface of the metal oxide layer. Preferably, prior to the annealing treatment, the first metallic layer includes gold and the second metallic layer includes nickel. After annealing the second metallic layer is substantially oxidized to nickel oxide, and a third metallic layer is formed over the metal oxide. Preferably, the third metallic layer is gold. In another embodiment, prior to the annealing treatment, three metallic layers are formed over the p-type semiconductor material. The first metallic layer has a first surface in contact with the p-type semiconductor layer; the second metallic layer has a first surface in contact with a second surface of the first material; and the third metallic layer has a surface in contact with a second surface of the second metallic material. In one embodiment, the third metallic layer is substantially the same metallic material as the first metallic layer. In a preferred embodiment, the first and third layers include gold and the second layer includes nickel which is substantially oxidized to nickel oxide during the annealing treatment.
The electrodes of the invention form ohmic contacts with p-type GaN-based semiconductor materials and typically are more light transmissive than prior art electrodes used to form ohmic contacts with p-type GaN-based semiconductor materials which do not contain metal oxides.